Millions of Americans already have been infected with swine flu. Forty-six states have widespread flu, and the president has declared a national emergency.

But only recently have U.S. health officials discovered why manufacturers can’t deliver as much swine flue vaccine as expected.

Officials at the Centers for Disease Control and Prevention say for months, the companies didn’t realize how far short their vaccine “yields” were falling. That’s because they didn’t have the chemicals — called reagents — that would have told them how much active ingredient they had in their vaccine production vats.